Once you're familiar with some scales, try this.
1:Turn on the drum machine or metronome - keep the tempo fairly slow to start with.
2: Pick a key and scale.
3:In time with the beat, play six random notes from the scale, then another seven random notes, and finally another six, all in immediate succession. Try to start and finish on the root note, without ever playing more than the limit of nineteen notes!
4:Keep practicing this exercise. As you become more fluent, try to concentrate on creating a structured melody as you play. If you like, you can pick up speed and/or experiment with other beats and time signiatures.

This exercise is a good first step to playing improvisational lead guitar. That said, it's probably best for jazz and blues guitarists, but there's nothing stopping you improvising in a fashion appropriate to rock, pop, funk, metal - whatever.